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Boron nitride: nanomaterial opportunities beyond graphene

Many posts in recent times have focussed on the potential of a new class of “wonder” materials collectively termed graphene. These materials are slowly transitioning from a research curiosity to a genuinely disruptive group of technologies that may change everyday products from water filtration to energy storage and beyond. The breadth of potential applications has meant extraordinary interest from the media and the public spurring a growing industry for graphene based products. The challenge remains though to scale graphene production to reduce costs without reducing quality.
According to a strict definition, graphene is a form of pure carbon that is only 1 atom thick yet 1000’s of those carbon atoms are connected together in an extended two-dimensional plane to form a solid material. Many production methods of graphene aspire to satisfy this definition, however, few actually achieve this goal. Often there may be some atomic impurities or defects in the plane or the thickness may be greater than 1 atom. Whilst not perfectly “graphene”, the material produced is close enough to have similar abilities to conduct electricity and heat better than anything material found before including metals and stronger too.
This combination of properties has seen graphene hyped as an excellent material to reinforce plastics making them not only stronger and lighter but also improve conductivity. Graphene reinforced plastics are hoped to drive change in the aerospace sector where composites are becoming more common to reduce weight and hence fuel use. One challenge remaining though is to mitigate the effects of lightning strikes on planes using plastic composites. Electrical conductivity is critical in achieving this and graphene composites may provide an answer.
For lightning strike protection, electrical conductivity is critical yet there are many other situations where an electrically conducting plastic is not ideal, in for example, adhesives used in parts of the electronics industry. The adhesive, though, should be strong and stable enough to withstand the high temperatures produced by the power consumed in the chips and to also conduct heat away. Here, graphene only satisfies two of the necessary factors for a suitable adhesive.
Interestingly, graphene is by no means the only nanomaterial with this kind of two dimensional structure and unique uses. The closest directly comparable material is hexagonal boron nitride (h-BN). Indeed such are the similarities to graphene, h-BN is sometimes referred to as white graphene. The two-dimensional form of h-BN has received very little attention in comparison to graphene but shares a similar physical structure and high conductivity of heat. However, unlike graphene h-BN is more or less electrically insulating.
The bulk or 3D form of h-BN has been used in thermal greases, thermal adhesives and thermal interface materials for many years. This is in large part due to the excellent thermal properties of high conductivity and low thermal expansion as well as being electrically insulating. So what then are the potential benefits to using a 2D form of h-BN instead?
The major benefit is the ability to use less h-BN in the adhesive to achieve the same or better thermal property enhancement. The reasons for this are two-fold. Firstly, the atomically thin form of h-BN has greater thermal conductance than the 3D form. This is a key example of the utility of 2D materials to replace existing 3D materials more generally in that their intrinsic properties are often better. Secondly, the adhesive reinforced with the 2D h-BN will be tougher as high particle concentrations using the 3D h-BN in the plastics can lead to embrittlement. Furthermore, the lower particle loadings will reduce the overall thermal expansion. In other words, as the adhesive gets hotter, the volume changes will be minimized reducing the potential for failure of the joint.
A second major advantage for single layer h-BN that is still under development is the ability to direct the heat in a given direction. Interestingly, the heat is conducted in the atomic plane of the particle and not out of the plane. This means that it should be possible to conduct heat to the edge of the adhesive connected to a heat sink rather than through the adhesive from the chip to the support if the orientation of the 2D particles is optimized.
The combination of the unique geometry of the nanoparticles and the high thermal conduction but low electrical conduction positions boron nitride as an interesting alternative to graphene or other mineral particles in many applications. The focus for this short post has been on enhancing the final properties of thermal adhesives, as like graphene, 2D h-BN is still finding the right market application and the potential in plastics and composites seem the most likely prospects in the near term. A critical challenge will be to scale manufacturing of atomically thin h-BN in order to put downward pressure on costs and make this technology viable.